Main heat exchanger and a process for cooling a tube side stream

ABSTRACT

A process for cooling a tube side stream in a main heat exchanger is described. The process comprises: a) supplying a first mass flow of a tube side stream to a first zone of individual tubes in the tube bundle; b) supplying a second mass flow of the tube side stream to a second zone of individual tubes in the tube bundle, the second zone being offset from the first zone; c) supplying a refrigerant stream on the shell side for cooling the first and second mass flows; d) removing the evaporated refrigerant stream from the warm end of the main heat exchanger; and, e) adjusting the first mass flow of the tube side stream relative to the second mass flow of the tube side stream to maximise the temperature of the removed evaporated refrigerant stream.

FIELD OF THE INVENTION

The present invention relates to a process for cooling a tube sidestream in a main heat exchanger. The present invention further relatesto a main heat exchanger for thermally processing a tube side stream.The present invention relates particularly though not exclusively to aprocess and a main heat exchanger for liquefying a gaseous, methane-richfeed to obtain a liquefied product known as “liquefied natural gas” or“LNG”.

Background to the Invention

A typical liquefaction process is described in U.S. Pat. No. 6,272,882in which the gaseous, methane-rich feed is supplied at elevated pressureto a first tube side of a main heat exchanger at its warm end. Thegaseous, methane-rich feed is cooled, liquefied and sub-cooled againstevaporating refrigerant to get a liquefied stream. The liquefied streamis removed from the main heat exchanger at its cold end and passed tostorage as liquefied product. Evaporated refrigerant is removed from theshell side of the main heat exchanger at its warm end. The evaporatedrefrigerant is compressed in at least one refrigerant compressor to gethigh-pressure refrigerant. The high-pressure refrigerant is partlycondensed and the partly condensed refrigerant is separated into aliquid heavy refrigerant fraction and a gaseous light refrigerantfraction. The heavy refrigerant fraction is sub-cooled in a second tubeside of the main heat exchanger to get a sub-cooled heavy refrigerantstream. The heavy refrigerant stream is introduced at reduced pressureinto the shell side of the main heat exchanger at an intermediate point,with the heavy refrigerant stream being allowed to evaporate in theshell side of the main heat exchanger. At least part of the lightrefrigerant fraction is cooled, liquefied and sub-cooled in a third tubeside of the main heat exchanger to get a sub-cooled light refrigerantstream. This light refrigerant stream is introduced at reduced pressureinto the shell side of the main heat exchanger at its cold end, and thelight refrigerant stream is allowed to evaporate in the shell side.

It is apparent from the description provided above that the tube side ofthe main heat exchanger is required to handle three streams, namely: i)a gaseous, methane-rich feed which enters the warm end of the first tubeside as a gas at elevated pressure, condenses as it travels through thefirst tube side, and leaves the cold end of the first tube side as asub-cooled liquefied stream; ii) a heavy refrigerant fraction whichenters the warm end of the second tube side as a liquid, is sub-cooledas it travels through the second tube side, and leaves the cold end ofthe second tube side as a sub-cooled heavy refrigerant stream; and, iii)a least a part of the light refrigerant fraction which enters the warmend of the third tube side as a vapour, is cooled, liquefied andsub-cooled as it travels through the third tube side, and leaves thecold end of the third tube side as a sub-cooled light refrigerantstream.

At the same time, the shell side of the main heat exchanger is requiredto handle: a) a heavy refrigerant stream which enters the shell side atan intermediate location (at a location referred to in the art as the“top of the warm tube bundle”), and which is evaporated within the shellside before being removed as a gas from the shell side at its warm end;and, b) a light refrigerant stream which enters the shell side atreduced pressure at its cold end (at a location referred to in the artas the “top of the cold tube bundle”), and which is evaporated withinthe shell side before being removed as a gas from the shell side at itswarm end.

Thus, in order to operate in the type of liquefaction process describedin U.S. Pat. No. 6,272,882, the main heat exchanger must be capable ofhandling both single and two phase streams, all of which condense atdifferent temperatures, with multiple tube-side and shell-side streamsbeing accommodated in the one exchanger. The main heat exchanger mustalso be capable of handling streams having a broad range of temperaturesand pressures. For this reason, the main heat exchanger used inliquefaction plants around the world is a “coil-wound” or “spiral-wound”heat exchanger.

In such coil-wound heat exchangers, the tubes for each of the individualstreams are distributed evenly in multiple layers which are wound arounda central pipe or mandrel to form a “bundle”. Each of the plurality oflayers of tubes may comprise hundreds of evenly sized tubes with an evendistribution of each of the first, second and third tube side fluids ineach layer in proportion to their flow ratios. The efficiency of themain heat exchanger relies on heat transfer between the shell side andthe tube side in each of these multiple layers being as balanced aspossible—both radially across the bundle and axially along the length ofthe bundle.

As spiral-wound heat exchangers become larger to perform increasedduties, it becomes increasingly difficult to distribute the shell sidefluids evenly. This is partly due to the fact that on the shell side,the composition of the heavy and light refrigerant streams changecontinuously along the length of the main heat exchanger as the lightcomponents boils off first. As a consequence, heat transfer between theshell side and each of the first, second and third tube sides may becomeuneven across the layers within the bundle. This uneven distribution oftemperature in the shell side fluids leads to unevenness in thetemperature in portions of each of the tube side fluids at the cold endsof the bundle from each layer of tubes in the bundle, and for theshell-side fluid exiting at the warm end.

When the system is in balance, the temperature difference between thetube sides and the shell side remains relatively constant but narrowalong the majority of the length of the main heat exchanger. When thesystem is out of balance, the close temperature differential between thetube sides and the shell side can become “pinched” at locations where avery small or no temperature differential exists at all. Such pinchingcauses a drop in efficiency of the main heat exchanger. A consequentialdrop in efficiency is also experienced in the associated mixedrefrigerant compression circuit which receives the fluid exiting thewarm end of the shell side of the main heat exchanger. If the main heatexchanger is working correctly, the fluid exiting the warm end of theshell side is a gas. When the main heat exchanger is out of balance, thefluid exiting the warm end of the shell side may comprise a two phasemixture of gas and liquid. Any liquid present represents a significantloss of efficiency and must also be removed to avoid potential damage tothe downstream refrigerant compression circuit.

The present invention provides a process and apparatus for improving theefficiency of a main heat exchanger by overcoming at least one of theproblems identified above.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is provided aprocess for cooling a tube side stream in a main heat exchanger having awarm end and a cold end, the main heat exchanger comprising a walldefining a shell side within which a coil-wound tube bundle is arrangedaround a central mandrel, the process comprising the steps of:

-   -   a) supplying a first mass flow of a tube side stream to the warm        end of a first zone of individual tubes in the tube bundle via a        first nozzle;    -   b) supplying a second mass flow of the tube side stream to the        warm end of a second zone of individual tubes in the tube bundle        via a second nozzle, the second zone being offset from the first        zone along a radius extending from the central mandrel to the        wall of the main heat exchanger;    -   c) supplying a refrigerant stream on the shell side for cooling        the first and second mass flows to form an evaporated        refrigerant stream;    -   d) removing the evaporated refrigerant stream from the warm end        of the main heat exchanger; and,    -   e) adjusting the first mass flow of the tube side stream        relative to the second mass flow of the tube side stream to        maximise the temperature of the evaporated refrigerant stream        removed in step d).

In one form, step e) comprises equalising the temperature of the firstmass flow of the tube side stream at a first axial location relative tothe length of the mandrel with the temperature of the second mass flowof the tube side stream at said first axial location by adjusting themass flow supplied to one or both of the first and second nozzles.

In one form, a first temperature sensor generates a first signalindicative of the temperature of the first mass flow and a secondtemperature sensor generates a second signal indicative of thetemperature of the second mass flow and step e) comprises using acontroller to adjust the first mass flow of the tube side streamrelative to the second mass flow of the tube side stream to equalise thefirst signal with the second signal. In one form, the first axiallocation is at or adjacent to the cold end of the main heat exchanger.In one form, the first zone is an inner zone of the tube bundle and thesecond zone is an outer zone of the tube bundle. In one form, whereinthe mass flow through the first nozzle is controllably adjusted using afirst valve and the mass flow through the second nozzle is controllablyadjusted using a second valve. In one form, one or both of the first andsecond valves is external to the main heat exchanger. In one form, oneor both of the first and second valves is a fail-safe open low pressuredrop valve. In one form, one or both of the first and second valves islocated at one or both of the warm end and the cold end of the tube sidestream.

In one form, the first nozzle supplies the tube fluid to the first zonevia a first tube sheet and the second nozzle supplies the tube sidefluid to the second zone via a second tube sheet. In one form, the tubebundle comprises a warm tube bundle arranged towards the warm end of themain heat exchanger, and a cold tube bundle arranged towards the coldend of the main heat exchanger, each of the warm tube bundle and thecold tube bundle having a warm end and a cold end and the first locationis at or adjacent to the cold end of the warm tube bundle. In one form,the tube side stream is a first tube side stream which enters the warmend of the warm tube bundle as a liquid and exits the cold end of thecold tube bundle as a sub-cooled liquid.

In one form, the first tube side stream enters the warm end of the warmtube bundle as a gaseous, methane-rich feed which has been liquefied bythe time it passes from the warm end of the warm tube bundle into thewarm end of the cold tube bundle. In one form, the first tube sidestream enters the warm end of the cold tube bundle as a liquid and exitsthe cold end of the cold tube bundle as a sub-cooled liquid. In oneform, wherein the sub-cooled liquid is removed from the cold end of thecold tube bundle of the main heat exchanger before being directed tostorage. In one form, the first tube side stream exchanges heat with apredominately liquid light refrigerant stream which is progressivelyboiled off on the shell side of the cold tube bundle. In one form,evaporated refrigerant removed from the warm end of the shell side ofthe main heat exchanger is fed to first and second refrigerantcompressors in which the evaporated refrigerant is compressed to form ahigh pressure refrigerant stream. In one form, the high pressurerefrigerant stream is directed to a heat exchanger in which it is cooledso as to produce a partly-condensed refrigerant stream which is thendirected in a separator to separate out a heavy refrigerant fraction inliquid form and a light refrigerant fraction in gaseous form.

In one form, the heavy refrigerant fraction becomes a second tube sidestream which is supplied at the warm end of the warm tube bundle as aliquid and exits at the cold end of the warm tube bundle as a sub-cooledheavy refrigerant stream in liquid form. In one form, the sub-cooledheavy refrigerant stream removed at the cold end of the warm tube bundleis expanded across a first expansion device to form a reduced pressureheavy refrigerant stream that is then introduced into the shell side ofthe main heat exchanger at a location intermediate between the cold endof the warm tube bundle and the warm end of the cold tube bundle, andwherein said reduced pressure heavy refrigerant stream is allowed toevaporate in the shell side, thereby cooling the fluids in the first,second and third tube side streams as they pass through the warm tubebundle. In one form, part of the light refrigerant fraction from theseparator becomes a third tube side stream which is introduced into thewarm end of the warm tube bundle as a gas and exits at the cold end ofthe cold tube bundle as a sub-cooled liquid. In one form, the third tubeside stream is cooled from a gas to a liquid as it passes through thewarm tube bundle and is cooled from a liquid to a sub-cooled liquid asit passes through the cool bundle.

In one form, the sub-cooled light refrigerant stream removed from thecold end of the cold tube bundle is expanded through a second expansiondevice to cause a reduction in pressure and produce a reduced pressurelight refrigerant stream. In one form, the reduced pressure lightrefrigerant stream is introduced into the shell side of the main heatexchanger at its cold end, and wherein said reduced pressure lightrefrigerant stream is allowed to evaporate in the shell side, therebycooling the fluids in the first and third tube side streams as theytravel through the cold tube bundle as well as providing cooling to thefluids in the first, second and third tube side streams as they travelthrough the warm tube bundle.

According to one aspect of the present invention there is provided amain heat exchanger for liquefying a tube side stream, the main heatexchanger having a warm end and a cold end in use, the main heatexchanger comprising:

-   -   a wall defining a shell side within which is arranged a        coil-wound tube bundle;    -   a first nozzle for supplying a first mass flow of a tube side        stream to the warm end of a first zone of individual tubes in        the tube bundle via a first nozzle;    -   a second nozzle for supplying a second mass flow of the tube        side stream to the warm end of a second zone of individual tubes        in the tube bundle via a second nozzle, the second zone being        offset from the first zone along a radius extending from the        central mandrel to the wall of the main heat exchanger;    -   a distributor for supplying a refrigerant stream on the shell        side for cooling the first and second mass flows to form an        evaporated refrigerant stream;    -   means for removing the evaporated refrigerant stream from the        warm end of the main heat exchanger; and,    -   a controller for adjusting the first mass flow of the tube side        stream supplied by the first nozzle relative to the second mass        flow of the tube side stream supplied by the second nozzle to        maximise the temperature of the evaporated refrigerant stream as        measured by a temperature sensor.

In one form, the controller adjusts the mass flow supplied to one orboth of the first and second nozzles to equalise the temperature of thefirst mass flow of the tube side stream at a first axial locationrelative to the length of the mandrel with the temperature of the secondmass flow of the tube side stream at said first axial location. In oneform, a first temperature sensor generates a first signal indicative ofthe temperature of the first mass flow and a second temperature sensorgenerates a second signal indicative of the temperature of the secondmass flow and the controller adjusts the first mass flow of the tubeside stream relative to the second mass flow of the tube side stream toequalise the first signal with the second signal. In one form, the firstaxial location is at or adjacent to the cold end of the main heatexchanger. In one form, the first zone is an inner zone of the tubebundle and the second zone is an outer zone of the tube bundle. In oneform, the mass flow through the first nozzle is controllably adjustedusing a first valve and the mass flow through the second nozzle iscontrollably adjusted using a second valve. In one form, one or both ofthe first and second valves is external to the main heat exchanger. Inone form, one or both of the first and second valves is a fail-safe openlow pressure drop valve. In one form, one or both of the first andsecond valves is located at one or both of the warm end and the cold endof the tube side stream. In one form, the first nozzle supplies the tubefluid to the first zone via a first tube sheet and the second nozzlesupplies the tube side fluid to the second zone via a second tube sheet.In one form, the tube bundle comprises a warm tube bundle arrangedtowards the warm end of the main heat exchanger, and a cold tube bundlearranged towards the cold end of the main heat exchanger, each of thewarm tube bundle and the cold tube bundle having a warm end and a coldend and the first location is at or adjacent to the cold end of the warmtube bundle.

According to a third aspect of the present invention there is provided aprocess for cooling a tube side stream in a main heat exchangersubstantially as herein described with reference to and as illustratedin FIGS. 2 and 3.

According to a fourth aspect of the present invention there is provideda main heat exchanger process for liquefying a tube side streamsubstantially as herein described with reference to and as illustratedin FIGS. 2 and 3.

DESCRIPTION OF THE DRAWINGS

In order to facilitate a more detailed understanding of the nature ofthe invention embodiments of the present invention will now be describedin detail, by way of example only, with reference to the accompanyingdrawings, in which:

FIG. 1 shows schematically the distribution of flows to each layer of aprior art spiral wound main heat exchanger;

FIG. 2 shows schematically a flow scheme of a plant for liquefyingnatural gas; and,

FIG. 3 shows schematically the distribution of flows to each layer ofthe main heat exchanger of one embodiment of the present invention.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Particular embodiments of the process and apparatus of the presentinvention are now described, with particular reference to a plant forliquefying a gaseous, methane-rich feed gas in the form of natural gasin a main heat exchanger to produced liquefied natural gas, by way ofexample only.

The present invention is equally applicable to a main heat exchangerused for other applications such as the production of ethylene or otherprocess requiring on two tube side streams instead of the three tubeside streams described in detail below. The terminology used herein isfor the purpose of describing particular embodiments only, and is notintended to limit the scope of the present invention. Unless definedotherwise, all technical and scientific terms used herein have the samemeanings as commonly understood by one of ordinary skill in the art towhich this invention belongs. In the drawings, it should be understoodthat like reference numbers refer to like parts.

Using a typical prior art spiral wound main heat exchanger such as theone illustrated schematically in FIG. 1, the tube bundle is spiral woundwhereby each tube side stream is introduced to the tube bundle via oneor more flow control nozzles arranged to evenly distribute the mass flowof any given type of tube side stream into a plurality of individualtubes arranged randomly yet evenly across the full radius of the tubebundle when viewed in cross-section. More specifically, each nozzlecauses the mass flow of each tube side stream to be distributed evenlybetween each layer of individual tubes within the tube bundle. When thetube bundle is wound so as to have a plurality of layers of individualtubes, the mass flow of any given tube side stream from any given nozzleis split evenly across each of the plurality of layers. The net resultis that every nozzle distributes an even amount of its mass flow acrossany given cross-section taken through the tube bundle—axially andradially. In an analogous manner, the mass flow of light refrigerantentering the shell side at the cold end of a cold tube bundle in themain heat exchanger is distributed across the shell side using a firstdistributor (not shown), and the mass flow of heavy refrigerant enteringthe shell side at the cold end of a warm tube bundle is distributedacross the shell side using a second distributor (not shown). This priorart arrangement is advocated for use in maintaining as even a heatbalance across the main heat exchanger as possible at all times.

The present invention is based in part on the realisation that it isdifficult to fix any imbalance in the temperature, composition or massflow rate distribution of the reduced pressure light and heavyrefrigerant streams on the shell side of the main heat exchanger. Whilstthe vapour phase present is capable of mixing in the radial direction tosome degree, the liquid phase present on the shell side does not to anysignificant extent with the result that any imbalance in temperatureacross the tube bundle cannot be corrected by making adjustments on theshell side. Instead, the Applicants have realised that an improvement inefficiency can be achieved by adjusting the mass flow of at least one ofthe tube side streams to compensate for any imbalance on the shell side.The present invention is further based in part on a realisation thatthis traditional method of construction of a spiral would heat exchangerprovides no mechanism to address problems which arise in the event of animbalance of cooling on the shell side of the main heat exchanger.

Using the process of the present invention, the tube bundle is wound insuch a way that any given nozzle supplies a tube side stream into onlyone zone of the tube bundle, each zone comprising a plurality of layersof individual tubes, so that the mass flow of that tube side stream toeach zone within the tube bundle can be separately controlled. Byproviding this level of control, the mass flow of each tube side streamto each zone of the bundle can be adjusted to compensate for an unevendistribution of cooling on the shell side, wherever and whenever thisoccurs. Advantageously the adjustable mass flows through each separatenozzle (and thus each separate zone) can also be used to redress heattransfer imbalance issues that might otherwise arise due to changes infeed gas composition over time, or from a change in vertical alignmentof the main heat exchanger, such as may occur on a vessel. In otherwords, the temperature of the evaporated refrigerant stream removed fromthe shell side at the warm end of the main heat exchanger is maximisedby separately adjusting the mass flow of the tube side stream in eachzone of the tube bundle as described in greater detail below. Anotherway to achieve maximum efficiency is to ensure that the exit temperatureof the tube side streams for each zone is as uniform as possible. Theoverarching aim is to match the tube side duty to the shell sideduty—even when the shell side duty is imbalanced.

FIGS. 2 and 3 illustrate one embodiment of a process or plant (10) forcooling a tube side stream in a main heat exchanger (12) according tothe present invention. The main heat exchanger (12) has a wall (14)defining a shell side (16) within which a coil-wound tube bundle (18) isarranged around a central mandrel (19), the main heat exchanger (12)having a warm end (20) and a cold end (22). A first mass flow (28) of atube side stream is supplied to the warm end (20) of a first zone (24)via a first nozzle (25). A second mass flow (30) of a tube side streamis supplied to the warm end (20) of a second zone (26) via a secondnozzle (27). The second zone (26) is offset from the first zone (24) aradius extending from the central mandrel (19) to the wall (14) of themain heat exchanger (12). In the embodiment illustrated in FIG. 3, thetube bundle (18) further includes an optional third intermediate zone(35) arranged between the first zone (24) and the second zone (26), saidthird zone (35) being supplied with a third mass flow (37) of the tubeside stream by the third nozzle (39). It is to be understood that anynumber of zones may be used provided only that supply to each zone iscontrolled by separate nozzles. It is to be further understood thatwithin each zone, the individual tubes remain evenly distributed and maybe arranged in a plurality of layers.

With reference to FIGS. 2 and 3, a single or mixed refrigerant stream(31) is introduced at the cold end (22) of the main heat exchanger andevaporated on the shell side (16) to provide cooling to the first andsecond mass flows (28 and 30, respectively) of the tube side stream. Anevaporated refrigerant stream (74) is removed from the warm end (20) ofthe main heat exchanger (12). The first mass flow (28) which flows onlythrough the first zone (24) is separately adjusted relative to thesecond mass flow (30) which flows only through the second zone (26) tomaximise the temperature of the evaporated refrigerant stream (74)removed from the warm end (20) of the main heat exchanger (12).

In one embodiment of the present invention, the temperature of theevaporated refrigerant stream (74) removed from the warm end (20) of themain heat exchanger (12) is maximised by equalising the temperature ofthe first mass flow (28) as measured at a first axial location (33)relative to the length of the mandrel (19) with the temperature of thesecond mass flow (30) as measured at said first axial location (33). Themass flow supplied by one or both of the first and second nozzles (25and 27, respectively) is adjusted in this way to ensure that thetemperature of said tube side stream in the first zone (24) is matchedto the temperature of said tube side stream in the second zone (26) atany give axial location along the length of the tube bundle (18). While,by way of example, it would be ideal for the exit temperature of thefirst mass flow (28) to be equal to the exit temperature of the secondmass flow (30) at the cold end (22) for maximum efficiency, the term“equalise” is used throughout this specification and the appended claimsto refer to incremental adjustment of at least one of the first andsecond mass flows (28 and 30, respectively) to achieve the result thatthe exit temperature of the first mass flow (28) more closely approachesthe exit temperature of the second mass flow (30) at the cold end (22).

In the embodiment illustrated in FIG. 3, the temperature of the firstmass flow (28) is measured using a first temperature sensor (32) withthe temperature of the second mass flow (30) being measured using asecond temperature sensor (34). With reference to FIG. 2, thetemperature of the evaporated refrigerant stream (74) removed from thewarm end (20) of the main heat exchanger (12) is measured using a thirdtemperature sensor (75).

For the purposes of automation of one embodiment of the process, a firstsignal (35) indicative of the temperature measured by the firsttemperature sensor (32) is compared with a second signal (41) indicativeof the temperature measured by the second temperature sensor (34) usinga controller (40). The controller (40) is then used to adjust the massflow supplied to the first zone (24) by the first nozzle (25) separatelyrelative to the mass flow supplied to the second zone (26) via thesecond nozzle (27) so as to equalise the first and second signals (35and 41). Alternatively or additionally, a third signal (77) indicativeof the temperature measured by the third temperature sensor (75) isprovided to the controller (40). The controller (40) is then used toadjust the mass flow supplied to the first zone (24) by the first nozzle(25) relative to the mass flow supplied to the second zone (26) via thesecond nozzle (27) so as to maximise the temperature of the evaporatedrefrigerant stream (74). When the tube bundle (18) further includes anoptional third intermediate zone (35), the controller (40) may receive afourth signal indicative of the temperature in the third intermediatezone in an analogous manner to allow adjustment of the third mass flow(37) supplied via the third nozzle (39).

It is to be understood that the total mass flow into the main heatexchanger (12) is controlled either upstream or downstream of the mainheat exchanger (12). Consequently, an adjustment made by controller (40)to any of the nozzles (25, 27 or 39) will change the relative mass flowthrough the other nozzles (25, 27 or 39) whilst the overall mass flowthrough the main heat exchanger remains constant.

In the embodiment illustrated in FIG. 3, each nozzle is provided with aflow valve, for example a low pressure butterfly valve, located eitherat the inlet or outlet of the tube side stream (either upstream ordownstream of the cold end of the tube bundle) to facilitate adjustmentof the mass flow through that nozzle. Thus, the mass flow through thefirst nozzle (25) is controllably adjusted using a first valve (45)while the mass flow through the second nozzle (27) is controllablyadjusted using a second valve (47). Advantageously, when one or both ofthe first and second valves (45 and 47, respectively) are external tothe main heat exchanger, adjustment of the mass flow rate through thefirst and second nozzles (25 and 27, respectively) can occur withouthaving to take the main heat exchanger offline, thus avoiding thedisruptive loss of production associated with shutdowns.

Reference is now made to FIG. 2 which illustrates schematically a plant(10) for liquefying a gaseous, methane-rich feed gas in the form ofnatural gas in a main heat exchanger (12). In this embodiment, the wall(14) of the main heat exchanger (12) defines a shell side (16) withinwhich is arranged two tube bundles, being a warm tube bundle (50) havinga warm end (52) and a cold end (54) and a cold tube bundle (56) having awarm end (58) and a cold end (60). The warm tube bundle (50) is arrangedtowards the warm end (20) of the main heat exchanger (12) and the coldtube bundle (56) is arranged towards the cold end (22) of the main heatexchanger (12). In the embodiment illustrated in FIG. 2, the tube bundleis arranged to received a first tube side stream (62), a second tubeside stream (64), and a third tube side stream (66) as described ingreater detail below. However, the present invention applies equally tomain heat exchanger operating with only one or two tube side streamsprovided only that a first mass flow of any given tube side stream isdirected to flow through a first subset of individual tubes and a secondmass flow of said tube side stream is directed to flow through a secondsubset of individual tubes, with each of the first and second subsets ofindividual tubes being radially offset across the coil-wound tubebundle.

In the embodiment illustrated in FIG. 2, the first tube side stream (62)enters the warm tube bundle (50) at elevated pressure as a gaseous,methane-rich feed which has been liquefied and partially sub-cooled bythe time it passes from the cold end (54) of the warm tube bundle (50)into the warm end (58) of the cold tube bundle (56). The first tube sidestream (62) enters the warm end (58) of the cold tube bundle (56) as apartially sub-cooled liquid and exits the cold end (60) of the cold tubebundle (56) as a further sub-cooled liquid. As it passes through thecold tube bundle (56), the first tube side stream (62) exchanges heatwith a predominately liquid light refrigerant stream (68) which isprogressively boiled off on the shell side (16) of the cold tube bundle(56). The resulting sub-cooled liquefied first tube side stream (70) isremoved from the cold end (22) of the main heat exchanger (12) beforebeing directed to storage (72).

An evaporated mixed refrigerant stream (74) removed from the shell side(16) at the warm end (20) of the main heat exchanger (12) is fed tofirst and second refrigerant compressors (76 and 78) in which theevaporated refrigerant stream (74) is compressed to form a high pressurerefrigerant stream (80). The high pressure refrigerant stream (80) isthen directed to one or more heat exchangers (82) in which it is cooledso as to produce a partly-condensed mixed refrigerant stream (84) whichis then directed in a separator (86) to separate out a heavy refrigerantfraction in liquid form (88) and a light refrigerant fraction in gaseousform (90). The heavy refrigerant fraction (88) becomes the second tubeside stream (64) which enters at the warm end (52) of the warm tubebundle (50) as a liquid and exits at the cold end (54) of the warm tubebundle (50) as a sub-cooled heavy refrigerant stream (92). In this way,the heavy refrigerant second tube side stream remains a liquid at alltimes as it passes through the warm tube bundle of the main heatexchanger.

The sub-cooled heavy refrigerant stream (92) removed at the cold end(54) of the warm tube bundle (50) is expanded across a first expansiondevice (94), for example a Joule-Thompson valve (“J-T valve”), to form areduced pressure heavy refrigerant stream (96) that is then introducedinto the shell side (16) of the main heat exchanger (12) at a locationintermediate between the cold end (54) of the warm tube bundle (50) andthe warm end (58) of the cold tube bundle (56). The reduced pressureheavy refrigerant stream (96) is thus one of the refrigerant streams(31) that is allowed to evaporate in the shell side (16), therebycooling the fluids in the first, second and third tube side streams (62,64 and 66, respectively) as they pass through the warm tube bundle (50).

Part of the light refrigerant fraction (90) from the separator (86)becomes the third tube side stream (66) which is introduced into thewarm end (52) of the warm tube bundle (50) as a gas and exits at thecold end (60) of the cold tube bundle (56) as a sub-cooled liquid lightrefrigerant stream (100). More specifically, the third tube side stream(66) is cooled from a gas to a liquid and partially sub-cooled as itpasses through the warm tube bundle (50) and is further cooled to asub-cooled liquid as it passes through the cool bundle (56). Thesub-cooled light refrigerant stream (100) removed from the cold end (22)of the main heat exchanger (12) is expanded through a second expansiondevice (102), for example a J-T valve to cause a reduction in pressureand produce a reduced pressure light refrigerant stream (104). Thereduced pressure light refrigerant stream (104) is thus another of therefrigerant streams (31) introduced into the shell side (16) of the mainheat exchanger (12). In this case, the reduced pressure lightrefrigerant stream (104) starts to evaporate in the shell side (16) toprovide cooling to the cold tube bundle (56), thereby cooling the fluidsin the first and third tube side streams (62 and 66, respectively) asthey travel through the cold tube bundle (56) as well as providingcooling to the fluids in the first, second and third tube side streams(62, 64 and 66, respectively) as they travel through the warm tubebundle (50).

When the process and apparatus of the present invention is used forliquefaction of a gaseous methane-rich feed to obtain a liquefiednatural gas, the tube side stream can be one or more of: the first tubeside stream; the second tube side stream; or, the third tube sidestream. The selection of which tube side stream(s) require rebalancingwill depend on the size of the temperature differentials measured fordifferent zones across the cold end of the tube bundle at the tube sidestream exits.

By way of example, the temperature of a first tube side stream exiting afirst zone at the cold end of the tube bundle may be compared with thetemperature of the first tube side stream exiting a second zone of thecold end of the tube bundle. In this example, the mass flow of the firsttube side stream into the warm end of the tube bundle is rebalanceduntil the temperature of the first tube side stream exiting the firstzone at the cold end of the tube bundle moves closer to the temperatureof the first tube side stream exiting the second zone at the cold end ofthe tube bundle. If the temperature of the first tube side streamexiting the first zone at the cold end of the tube bundle is higher thanthe temperature of the first tube side stream exiting the second zone atthe cold end of the tube bundle, the step of rebalancing of the massflow is achieved by restricting the flow of the first tube side streamto the first zone at the warm end of the tube bundle. In this way, themass flow of the first tube side stream to the second zone at the warmend of the tube bundle is essentially increased as the overall mass flowrate of the first tube side stream into the warm end of the tube bundledoes not change.

Analogously, by way of further example, the temperature of the secondtube side stream exiting a first zone at the cold end of the warm tubebundle may be compared with the temperature of the second tube sidestream exiting a second zone at the cold end of the warm tube bundle. Inthis example, the mass flow of the second tube side stream into the warmend of the warm tube bundle is rebalanced until the temperature of thesecond tube side stream exiting the first zone at the cold end of thewarm tube bundle moves closer to being equal to the temperature of thesecond tube side stream exiting the second zone at the cold end of thewarm tube bundle. If the temperature of the second tube side streamexiting the first zone at the cold end of the warm tube bundle is lowerthan the temperature of the second tube side stream exiting the secondzone at the cold end of the warm tube bundle, the step of rebalancing ofthe mass flow is achieved by restricting the flow of the second tubeside stream to the second zone at the warm end of the warm tube bundle.In this way, the mass flow of the second tube side stream to the firstzone at the warm end of the warm tube bundle is essentially increased asthe overall mass flow rate of the second tube side stream into the warmend of the warm tube bundle does not change.

Restriction of the mass flow of a tube side stream to any given zonewithin the bundle can be achieved by adjusting the mass flows throughthe nozzle or valve responsible for directing the mass flow of that sidestream to said zone. It is considered a matter of routine for a personskilled in the art to determine the degree to which flow through anozzle needs to be adjusted for any given zone of the tube bundle tocompensate for the difference in temperature of said tube side streamexiting the cold end of the tube bundle for said zone. This can beachieved using modelling techniques well known in the art.

Now that embodiments of the invention have been described in detail, itwill be apparent to persons skilled in the relevant art that numerousvariations and modifications can be made without departing from thebasic inventive concepts. For example, a plurality of shell sidetemperature sensors (71) may be used to provide a correspondingplurality of signals indicative of the temperature of each zone withinthe tube bundle. This plurality of signals may be fed to the controller(40) to facilitate controlled adjustment of the mass flow of a tube sidestream to said zones. All such modifications and variations areconsidered to be within the scope of the present invention, the natureof which is to be determined from the foregoing description and theappended claims.

Any patents cited in this specification, are herein incorporated byreference. It will be clearly understood that, although a number ofprior art publications are referred to herein, this reference does notconstitute an admission that any of these documents forms part of thecommon general knowledge in the art, in Australia or in any othercountry. In the summary of the invention, the description and claimswhich follow, except where the context requires otherwise due to expresslanguage or necessary implication, the word “comprise” or variationssuch as “comprises” or “comprising” is used in an inclusive sense, i.e.to specify the presence of the stated features but not to preclude thepresence or addition of further features in various embodiments of theinvention.

1. A process for cooling a tube side stream in a main heat exchangerhaving a warm end and a cold end, the main heat exchanger comprising awall defining a shell side within which a coil-wound tube bundle isarranged around a central mandrel, the process comprising the steps of:a) supplying a first mass flow of a tube side stream to the warm end ofa first zone of individual tubes in the tube bundle via a first nozzle;b) supplying a second mass flow of the tube side stream to the warm endof a second zone of individual tubes in the tube bundle via a secondnozzle, the second zone being offset from the first zone along a radiusextending from the central mandrel to the wall of the main heatexchanger; c) supplying a refrigerant stream on the shell side forcooling the first and second mass flows to form an evaporatedrefrigerant stream; d) removing the evaporated refrigerant stream fromthe warm end of the main heat exchanger; and, e) adjusting the firstmass flow of the tube side stream relative to the second mass flow ofthe tube side stream to maximise the temperature of the evaporatedrefrigerant stream removed in step d) wherein step e) comprisesequalizing the temperature of the first mass flow of the tube sidestream at a first axial location relative to the length of the mandrelwith the temperature of the second mass flow of the tube side stream atsaid first axial location by adjusting the mass flow supplied to one orboth of the first and second nozzles.
 2. (canceled)
 3. The process ofclaim 1 wherein a first temperature sensor generates a first signalindicative of the temperature of the first mass flow and a secondtemperature sensor generates a second signal indicative of thetemperature of the second mass flow and step e) comprises using acontroller to adjust the first mass flow of the tube side streamrelative to the second mass flow of the tube side stream to equalise thefirst signal with the second signal.
 4. The process of claim 1 whereinthe first axial location is at or adjacent to the cold end of the mainheat exchanger.
 5. The process of claim 1 wherein the first zone is aninner zone of the tube bundle and the second zone is an outer zone ofthe tube bundle.
 6. The process of claim 1 wherein the mass flow throughthe first nozzle is controllably adjusted using a first valve and themass flow through the second nozzle is controllably adjusted using asecond valve.
 7. The process of claim 6 wherein one or both of the firstand second valves is external to the main heat exchanger.
 8. The processof claim 6 wherein one or both of the first and second valves is afail-safe open low pressure drop valve.
 9. The process of claim 6wherein one or both of the first and second valves is located at one orboth of the warm end and the cold end of the tube side stream.
 10. Theprocess of claim 1 wherein the first nozzle supplies the tube fluid tothe first zone via a first tube sheet and the second nozzle supplies thetube side fluid to the second zone via a second tube sheet.
 11. Theprocess of claim 1 wherein the tube bundle comprises a warm tube bundlearranged towards the warm end of the main heat exchanger, and a coldtube bundle arranged towards the cold end of the main heat exchanger,each of the warm tube bundle and the cold tube bundle having a warm endand a cold end and the first location is at or adjacent to the cold endof the warm tube bundle.
 12. The process of claim 11 wherein the tubeside stream is a first tube side stream which enters the warm end of thewarm tube bundle as a liquid and exits the cold end of the cold tubebundle as a sub-cooled liquid.
 13. The process of claim 11 wherein thefirst tube side stream enters the warm end of the warm tube bundle as agaseous, methane-rich feed which has been liquefied by the time itpasses from the warm end of the warm tube bundle into the warm end ofthe cold tube bundle.
 14. The process of claim 13 wherein the first tubeside stream enters the warm end of the cold tube bundle as a liquid andexits the cold end of the cold tube bundle as a sub-cooled liquid. 15.The process of claim 14 wherein the sub-cooled liquid is removed fromthe cold end of the cold tube bundle of the main heat exchanger beforebeing directed to storage.
 16. The process of claim 15 wherein the firsttube side stream exchanges heat with a predominately liquid lightrefrigerant stream which is progressively boiled off on the shell sideof the cold tube bundle.
 17. The process of claim 16 wherein evaporatedrefrigerant removed from the warm end of the shell side of the main heatexchanger is fed to first and second refrigerant compressors in whichthe evaporated refrigerant is compressed to form a high pressurerefrigerant stream.
 18. The process of claim 17 wherein the highpressure refrigerant stream is directed to a heat exchanger in which itis cooled so as to produce a partly-condensed refrigerant stream whichis then directed in a separator to separate out a heavy refrigerantfraction in liquid form and a light refrigerant fraction in gaseousform.
 19. The process of claim 18 wherein the heavy refrigerant fractionbecomes a second tube side stream which is supplied at the warm end ofthe warm tube bundle as a liquid and exits at the cold end of the warmtube bundle as a sub-cooled heavy refrigerant stream in liquid form. 20.The process of claim 19 wherein the sub-cooled heavy refrigerant streamremoved at the cold end of the warm tube bundle is expanded across afirst expansion device to form a reduced pressure heavy refrigerantstream that is then introduced into the shell side of the main heatexchanger at a location intermediate between the cold end of the warmtube bundle and the warm end of the cold tube bundle, and wherein saidreduced pressure heavy refrigerant stream is allowed to evaporate in theshell side, thereby cooling the fluids in the first, second and thirdtube side streams as they pass through the warm tube bundle.
 21. Theprocess of claim 20 wherein part of the light refrigerant fraction fromthe separator becomes a third tube side stream which is introduced intothe warm end of the warm tube bundle as a gas and exits at the cold endof the cold tube bundle as a sub-cooled liquid.
 22. The process of claim21 wherein the third tube side stream is cooled from a gas to a liquidas it passes through the warm tube bundle and is cooled from a liquid toa sub-cooled liquid as it passes through the cool bundle.
 23. Theprocess of claim 22 wherein the sub-cooled light refrigerant streamremoved from the cold end of the cold tube bundle is expanded through asecond expansion device to cause a reduction in pressure and produce areduced pressure light refrigerant stream.
 24. The process of claim 23wherein the reduced pressure light refrigerant stream is introduced intothe shell side of the main heat exchanger at its cold end, and whereinsaid reduced pressure light refrigerant stream is allowed to evaporatein the shell side, thereby cooling the fluids in the first and thirdtube side streams as they travel through the cold tube bundle as well asproviding cooling to the fluids in the first, second and third tube sidestreams as they travel through the warm tube bundle.
 25. (canceled) 26.(canceled)
 27. (canceled)
 28. (canceled)
 29. (canceled)
 30. (canceled)31. (canceled)
 32. (canceled)
 33. (canceled)
 34. (canceled) 35.(canceled)
 36. (canceled)
 37. (canceled)